Internal combustion engines must be controlled in many different ways to provide acceptable driving comfort during all operating conditions. Some methods use engine output, or torque, control where the actual engine torque is controlled to a desired engine torque through an output adjusting device, such as with an electronic throttle, ignition timing, or various other devices. In some cases, such as during normal driving conditions, the desired engine torque is calculated from the amount of depression of an accelerator pedal. In other conditions, such as idle speed control, the desired engine torque is calculated based on a speed error between actual engine speed and a desired engine speed. Some attempts have been made to use this torque control architecture to improve driveability during deceleration conditions, such as when a driver releases their foot to the minimum accelerator pedal position, known to those skilled in the art as a tip-out. During a tip-out, the driver is indicating a desire for reduced engine output.
One system that attempts to use speed control during deceleration conditions operates the engine in such a way as to maintain constant engine speed during slow moving or stopped conditions. In this system, the engine is controlled to a constant speed taking into account the loading from the torque converter. The loading from the torque converter is calculated based on the engine speed and turbine speed. Engine speed can be controlled to a constant level during deceleration to adsorb energy from the vehicle and assists in vehicle braking. Further, as turbine speed increases, the desired engine speed is reduced to provide even more engine braking. Such a system is described in DE 4321413A1.
The inventors herein have recognized a disadvantage with the above approach. In particular, when the accelerator pedal is released and subsequently engaged, the prior art system exhibits poor driveability due transmission gears lash. For example, when the engine transitions from exerting a positive torque to exerting a negative torque (or being driven), the gears in the transmission separate at the zero torque transition point. Then, after passing through the zero torque point, the gears again make contact to transfer torque. This series of events produces an impact, or clunk, resulting in poor driveability and customer dissatisfaction. In other words, the engine first exerts a positive torque through the torque converter onto the transmission input gears to drive the vehicle. Then, when using the prior art approach during deceleration, the engine is driven by the torque from the transmission through the torque converter. The transition between these to modes is the point where the engine is producing exactly zero engine brake torque. Then, at this transition point, the gears in the transmission separate because of inevitable transmission gear lash. When the gears again make contact, they do so dynamically resulting in an undesirable impact.
This disadvantage of the prior art is exacerbated when the operator returns the accelerator pedal to a depressed position, indicating a desire for increased engine torque. In this situation, the zero torque transition point must again be traversed. However, in this situation, the engine is producing a larger amount of torque than during deceleration because the driver is requesting acceleration. Thus, another, more severe, impact is experienced due to the transmission lash during the zero torque transition.